Allocating priority levels in a data flow

ABSTRACT

A method of allocating a priority level in an information flow in a data transmission system including receiving a plurality of data packets and allocating a priority level to each data packet is provided with a controller employing such method. The priority level may be determined in accordance with determining a class of loss of the data packet and a class of service for the data packet. The priority level is also a function of the current load of the information flow. It has been established that the choice for each degree of freedom should preferably be made independently and concurrently. The invention, therefore, provides a method for performing such a composite choice.

CROSS-REFERENCE TO FOREIGN APPLICATION

[0001] This application is the National Stage of, and therefore claimsthe benefit of, International Application No. PCT/GB00/03785 filed onOct. 3, 2000, entitled “ALLOCATING PRIORITY LEVELS IN A DATA FLOW,”which was published in English under International Publication Number WO02/30062 on Apr. 11, 2002 and is incorporated herein by reference in itsentirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] Serial No. Title Inventor(s) Filing Date [U4EA-0002] DATA FLOWCONTROL Davies, et Concurrent al. Herewith [U4EA-0003] PACKET SEQUENCEDavies, et Concurrent CONTROL al. Herewith [U4EA-0004] INFORMATION FLOWDavies, et Concurrent CONTROL IN A PACKET al. Herewith NETWORK BASED ONVARIABLE CONCEPTUAL PACKET LENGTHS [U4EA-0005] FILTERING DATA Davies, etConcurrent FLOWS al. Herewith [U4EA-0006] PRIORITIZING DATA Davies, etConcurrent WITH FLOW CONTROL al. Herewith [U4EA-0007] POLICING DATABASED Davies, et Concurrent ON DATA LOAD al. Herewith PROFILE

[0003] The above-listed non-provisional applications are commonlyassigned with the present invention and are incorporated herein byreference in their entirety.

TECHNICAL FIELD OF THE INVENTION

[0004] The present invention is directed, in general, to transmission ofdigital packets in telecommunications systems and, more specifically, tocontention management within and between streams of packets in store andforward networks.

BACKGROUND OF THE INVENTION

[0005] There exist many techniques for controlling digital packetinformation flows. Some such techniques involve providing a certainquality of service for different types of traffic. There are a number ofgeneral requirements associated with ensuring quality of service levels.Where quality differentiation is provided for more than one data stream,it is preferable to ensure that different quality requirements for thevarious data streams of packets are individually and collectively metwithin the constraint of the finite quality available. In order toprovide differential levels of loss and delay as well as throughput, thequantity of input data and its temporal pattern should be constrained.The quantity of input data serviced, i.e. the throughput of data, isknown as long term management. Controlling the temporal pattern of datais known as short term management. The traffic in particular datastreams also should be protected from the consequential effects ofburstiness in other data streams. For example, individual data streamsof traffic should be protected from the effects of protocols such asTransmission Control Protocol (TCP), which is designed to use as muchbandwidth as possible without regard to other data streams, and frommalicious intentions or errors in the end devices of a network. It isalso preferable to manage the interleaving of individual data streamswithin the constraints of the available resources.

[0006] The management of quality of service is particularly difficult atthe periphery or edge of the network, as the devices are not under thecontrol of the central administrator of the network. The behaviour ofthese devices, therefore, cannot be assumed or predicted. Theever-increasing diversity in applications, traffic, and convergence alsocomplicates the management of quality of service. Different traffic hasdifferent quality of service requirements, and the consequences of delayand/or loss differs for different traffic according to theinterpretation associated with an application.

[0007] The possibility of replicating network devices to enable trafficwith different quality requirements to be physically separated andprocessed separately is impractical, as the implementation of networkdevices is expensive. For this reason, it is desirable to manage qualityof service of different traffic using a single network device.

[0008] There is currently a trend towards forming traffic patterns asconstant bit rate patterns, with the aim of increasing predictability.This is deterministic control which focuses on improving the losscharacteristics and efficiency of the network. However, such techniqueshave the disadvantage that quality assurance in the presence of“over-booking” often requires total global knowledge of the behavior ofthe sources in the network such as on/off times, relative phase, and theeffects of past history on the state of the network.

[0009] The network may employ finite queuing systems for assistance intraffic management. Recent research in quality of service has shown thatthere are two degrees of freedom in the finite queuing systems. Theallocation of the individual degrees to an individual packet, however,has not been the subject of previous study.

[0010] Accordingly, what is needed in the art is an improved techniquefor controlling information flow in a data transmission system, whichenables the control of the quality of service requirements for differenttypes of traffic to be improved.

SUMMARY OF THE INVENTION

[0011] To address the above-discussed deficiencies of the prior art, thepresent invention provides a method of allocating a priority level in aninformation flow in a data transmission system including receiving aplurality of data packets and allocating a priority level to each datapacket. The priority level may be determined in accordance withdetermining a class of loss of the data packet and a class of servicefor the data packet. The priority level is also a function of thecurrent load of the information flow. It has been established that thechoice for each degree of freedom should preferably be madeindependently and concurrently. The invention, therefore, provides amethod for performing such a composite choice.

[0012] The method may further include generating a modified informationflow by providing a variable spacing between the data packets. Thevariable spacing may be random or pseudo-random. The priority levels mayalso be allocated in accordance with a state of a queue that the datapackets are selectively admitted. Each data packet may be selectivelyadmitted to at least one of a plurality of queues.

[0013] The present invention also provides a controller for allocating apriority level in an information flow including a means for allocating apriority level to each data packet. The means for allocating a prioritylevel includes determining a class of loss for the data packet anddetermining a class of urgency for the data packet. The priority levelis further determined in accordance with a current load of theinformation flow.

[0014] The means for allocating a priority level may be embodied atleast in part in a policer/shaper. A plurality of parallel controllersmay be provided for generating a plurality of information flows. Amultiplexor may be provided for multiplexing the information flows.

[0015] The foregoing has outlined preferred and alternative features ofthe present invention so that those skilled in the art may betterunderstand the detailed description of the invention that follows.Additional features of the invention will be described hereinafter thatform the subject of the claims of the invention. Those skilled in theart should appreciate that they can readily use the disclosed conceptionand specific embodiment as a basis for designing or modifying otherstructures for carrying out the same purposes of the present invention.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For a more complete understanding of the present invention,reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which:

[0017]FIG. 1 illustrates a block diagram of an architecture of amultiplexer constructed in accordance with the principles of theinvention;

[0018]FIG. 2 illustrates a diagram representing queuing in accordancewith the principles of the invention;

[0019]FIG. 3 illustrates a diagram representing allocating prioritylevels in accordance with the principles of the invention;

[0020]FIG. 4 illustrates a diagram representing stochastically servicingpackets in accordance with the principles of the invention;

[0021] FIGS. 5(a), 5(b) and 5(c) illustrate diagrams representingconceptual packet length in accordance with the principles of theinvention.

[0022] FIGS. 6(a) and 6(b) illustrate a block diagram of an embodimentof cascading policer/shapers constructed in accordance with theprinciples of the invention;

[0023]FIG. 7 illustrates a diagram defining a transported load as afunction of an offered load in accordance with the principles of thepresent invention;

[0024]FIG. 8 illustrates a block diagram of an embodiment of apolicer/shaper constructed in accordance with the principles of thepresent invention;

[0025]FIG. 9 illustrates a block diagram of an embodiment of acherish/urgency multiplexor constructed in accordance with theprinciples of the present invention; and

[0026]FIG. 10 illustrates a graph representing a load profile for apolicer/shaper implementation in accordance with the principles of thepresent invention.

DETAILED DESCRIPTION

[0027] Referring initially to FIG. 1, illustrated is a block diagram ofan embodiment of an architecture of a multiplexor or controller,generally designated 100, constructed in accordance with the principlesof the present invention. It should be noted that the application of thepresent invention is not limited to the specific architecture shown inFIG. 1 and from reading the following description one skilled in the artwill appreciate the general applicability of the present invention. Themultiplexor 100 multiplexes streams of data in a packet-based network aswill be described in further detail hereinbelow.

[0028] The multiplexor 100, includes an input interface 101, a streamidentifier 102, a plurality of policer/shapers 104 a, 104 b, 104 c, 104d, which are collectively referred to as policer/shapers 104, acherish/urgency multiplexor 106, a rate limiter 108, an output interface110, a queue memory 112, and a queue memory manager 114.

[0029] The multiplexor 100 receives an information flow as an input online 116. The information flow may include packets which may beassociated with various different types of data traffic. For example,the packets may be received in parallel or in series. The input on theline 116 may receive parallel information flows. For the purposes of thedescribed example, the information flow, or data stream, may include aVoice over Internet Protocol (VoIP) stream, a block data transferstream, and a telnet traffic stream.

[0030] The input interface 101 provides the basic functionalitynecessary to receive transmitted data packets from an external system ordevice via a transmission medium. The input interface 101 receives thedata packets from the data stream on the input line 116 and forwards thereceived data packets, or simply packets, preferably in parallel, onlines 118 to both the stream identifier 102 and the queue memory manager114. The input interface 101 should have functionality which isappropriate for the particular external system or device from which thepackets on the data stream originate. The input interface 101 isgenerally not directly responsible for any processing related to thequality management of the data stream in accordance with the presentinvention.

[0031] The input interface 101 may have some basic functionality toperform an initial check on the packets received in the data stream. Forexample, if a corrupted packet is received the input interface 101 maydiscard the packet. The structure of the input interface 101 isimplementation dependent. The basic functionality of the input interface101 that performs any desired features will be apparent to one skilledin the art.

[0032] The queue memory manager 114 is responsible for managing a set ofqueues and a packet storage area. The queue memory manager 114 receivesthe packets from the input interface 101. On arrival of the packets, thequeue memory manager 114 allocates a unique identifier to each of thepackets, and sends the packet identifier for each of the packets to thestream identifier 102 on line 140. The queue memory manager 114additionally stores the packets in a temporary packet buffer. The queuememory manager 114 also assigns a reference counter to each of thepackets and initializes the reference counter for one of the packets tozero. As will be described further hereinbelow, the reference counter isused by the queue memory manager 114 to determine whether the one packetassociated with the reference counter is still being processed by themultiplexor 100, or whether it should be removed from (or not initiallyentered into) the queue memory 112.

[0033] The packet identifier is an identifier which uniquely identifiesa packet. The packet identifier uniquely identifies the packet for thepurpose of storing the packet identifier in the queue memory 112 as isdescribed further hereinbelow and distinguishing each packet from otherpackets in the queue memory 112. In one implementation, the packetidentifier is a number and each packet may be allocated a packetidentifier that is the next number in a sequence. Alternatively, thepacket identifier may be composed of unique information from a packetheader. In systems in which size of the packet is variable, the lengthof the packet may be included in the packet identifier.

[0034] The stream identifier 102 also receives the packets from theinput interface 101 on lines 118, and receives the packet identifier foreach packet on line 140 from the queue memory manager 114. The streamidentifier 102 is responsible for determining which data stream each ofthe packets belong. Thus, for example, the stream identifier 102 willdetermine whether a particular packet is associated with a VoIP stream,a block data transfer stream, or a telnet traffic stream. In accordancewith the data stream to which the particular received packet identifierbelongs, the stream identifier 102 forwards the packet identifier forthat packet to one of the policer/shapers 104 for further processing.

[0035] As will become apparent from the following description, theremainder of the processing is based on the packet identifier and notthe packet. The packet identifier advantageously provides an efficientrepresentation of the packet. As will be described hereinafter, thequeue memory 112 and the packet identifiers ensure that the originalsequence position of each individual packet is not lost in themultiplexing operation.

[0036] Policer/shapers 104 are an operational variant of a First-In,First-Out (FIFO) queue, in which there is subsidiary processingassociated with the insertion and removal of elements from the queue.Such FIFO queues are well-known, and their implementation will be wellwithin the scope of a person skilled in the art. The configuration ofthe policer/shapers 104 is implementation dependent.

[0037] In FIG. 1, one policer/shaper 104 may be allocated to each of thedifferent types of data streams being received. Thus, for example, thepolicer/shaper 104 a may be allocated to a VoIP stream, thepolicer/shaper 104 b may be allocated to a block data transfer stream,and the policer/shaper 104 c may be allocated to a telnet trafficstream. The policer/shaper 104 d, coupled to input line 120 d and outputline 122 d, is shown in FIG. 1 by way of illustrating a means formanaging packets associated with streams other than those processed bythe policer/shapers 104 a, 104 b, 104 c. Thus the stream identifier 102forwards packet identifiers associated with VoIP packets on input line120 a to the policer/shaper 104 a, forwards packet identifiersassociated with block data transfer packets on input line 120 b to thepolicer/shaper 104 b, and forwards packet identifiers associated withtelnet traffic packets on input line 120 c to the policer/shaper 104 c.

[0038] In the case of multi-casting or other services implementingreplication (e.g., monitoring), the stream identifier 102 may forwardpacket identifiers to more than one policer/shaper 104. Additionally,the stream identifier 102 does not forward packets for furtherprocessing within the multiplexor 100. Rather, the stream identifier 102forwards the packet identifiers allocated to packets for furtherprocessing in the multiplexor 100.

[0039] The policer/shapers 104 are responsible for the assignment ofquality classifications to the packets within the data stream, andresponsible for the quality control of the data stream in both the shortterm and the long term by selectively discarding packet identifiers inthe data stream and selectively time-shifting packets in the datastream. One function of the policer/shapers 104 is to service packets,using the corresponding packet identifiers, with a variable servicerate. As a result, the packet identifiers leaving the policer/shapers104 are variably spaced, preferably with a random or pseudo-randomspacing. Spacing the packet identifiers randomly ensures that the leasturgent traffic is eventually serviced by the cherish/urgency multiplexor106. The variable spacing of the packets at output lines 122 a, 122 b,122 c, 122 d, (which are collectively referred to as output lines 122)of the policer/shapers 104 reduces the coherence between streams fromindependent sources. Creating independent temporal patterns betweenstreams increases fairness of the cherish/urgency multiplexor 106decision processes. Cherish/urgency classifications allocated to thepacket identifiers in the policer/shapers 104 do not on their own ensurefairness unconditionally.

[0040] For example, the delay experienced by a packet with an associatedlow urgency level depends on the temporal pattern of more urgent trafficstreams. If two streams with different cherish/urgency levels aretemporarily coherent, the more cherished packets may always “win” therace to enter the cherish/urgency multiplexor 106 and/or the more urgentdata may always be transmitted first. This becomes statistically lesslikely if the streams have variably spaced packets.

[0041] Referring now to FIG. 4, illustrated is a diagram representingstochastically servicing packets in accordance with the principles ofthe present invention. FIG. 4 illustrates the policer/shapers 104 a, 104b, of FIG. 1 with examples of the data streams on the input lines 120 a,120 b, and the output lines 122 a, 122 b. The policer/shaper 104 areceives packet identifiers 200 a, 200 b, 200 c, at a constant spacing“t” on the input line 120 a. The policer/shaper 104 b receives packetidentifiers 202 a, 202 b, 202 c, at a constant spacing “t” on the inputline 120 b. The policer/shaper 104 a services the packet identifiers inits queue such that the packets identifiers 200 a, 200 b, 200 c, aregenerated on the output line 122 a with variable spacing therebetween.Similarly, the servicing of packet identifiers 202 a, 202 b, 202 c, bythe policer/shaper 104 b results in packets identifiers 202 a, 202 b,202 c, on the output line 122 b having variable spacing.

[0042] A second function of the policer/shapers 104 is to limit thevolume of traffic on a data stream which may be achieved by theselective discard of packets. The nature of the policing policy isspecified in terms of a load profile, which is a function of (at least)the offered load (i.e., an arrival rate of packets on a data stream).Therefore, and advantageously, there can be different characteristicsdefined for various load levels. That is, quality can be down-graded orup-graded as the arrival rate increases. Transport characteristics can,therefore, be configured to better match the application requirementsand to avoid effects of an abnormal offered load (e.g., denial ofservice attacks).

[0043] Referring to FIG. 7, the principle of defining transportcharacteristics (e.g., transport loads) in terms of an offered load isillustrated. In FIG. 7, the x-axis illustrates the offered load and they-axis illustrates a load which is actually transported. As can be seenin FIG. 7, there are two threshold levels, namely, a lower thresholdlevel 250 and an upper threshold level 252. If the offered load does notexceed the upper threshold level 252, then the offered load can betransported within configured boundaries. If the offered load exceedsthe upper threshold level 252, then the offered load cannot beguaranteed to be transmitted. The lower threshold level 250 representsthe minimum transported load regardless of how high the offered loadbecomes.

[0044] Thus, once the offered load reaches the upper threshold level 252and point 254, the transported load is adjusted such that it is reduced.As the offered load increases, the transported load continues to reduceto the point where it steadies out and tends towards the lower thresholdlevel 250.

[0045] Thus, the rate of packet discard may be determined by the offeredload. The upper threshold level 252 defines a level at which theselective discard of packets is triggered. Packets are preferablydiscarded based on an instantaneous approximation of the offered load.Packets may be discarded probabilistically based on an instantaneousapproximation of the offered load. If the offered load results in atransported load exceeding the upper threshold level 252, thetransported load is reduced below the upper threshold level 252 byselectively discarding packets.

[0046] When reduced, the transported load is preferably reduced to alevel above the lower threshold level 250. The reduction in thetransported load is greater, the larger the offered load. Thetransported load is further reduced responsive to an increase in theoffered load.

[0047] A policer/shaper, such as the policer/shapers 104, achieves adesired load profile by a selective discard of packets. Packets arepreferably discarded based on an instantaneous approximation of theoffered load. Referring to FIG. 10, illustrated is a graph representinga load profile for a policer/shaper implementation in accordance withthe principles of the present invention. FIG. 10 illustrates a loadprofile for an exemplary policer/shaper implementated as a queue with 10buffers and stochastic service times sampled from an exponentialdistribution whose rate parameter is dependent on the number of packetsqueuing. Service rate parameters, e.g., μ₁=μ₂= . . . =μ₉=1.3234,μ₁₀=0.5, represent the numeric values for the service rates and theoffered and transported loads are scaled so that the upper thresholdlevel takes the value 1. A description of such an implementation isprovided in an example hereinafter. The load profile illustrated in FIG.10 depicts the average transported load when the arriving traffic has aPoisson pattern. The results associated with application of standardqueuing theory permit the determination of the expected loss rate.

[0048] Returning now to FIG. 1, a third function of the policer/shapers104 is to allocate cherish and urgency classifications to the packetidentifiers included in their queues. By allocating such classificationsto the packet identifiers, the packets themselves are inherentlyallocated the same classification. The principle of cherish and urgencylevels is discussed in International Patent Application No.PCT/GB00/01569. A cherish level indicates a class or level of loss for apacket. The class of loss indicates the tendency of the packet to bediscarded. An urgency level indicates the level of urgency by which apacket should be processed. The urgency and cherish levels can, incombination, be considered to constitute a priority level for a packet.Thus, a priority level, for example, has two components in this context.

[0049] As discussed hereinabove, in the policer/shaper 104, transportcharacteristics for a stream, including allocation of cherish andurgency levels, are determined based on the offered load. The cherishand urgency classifications are advantageously assigned simultaneouslyto packet identifiers based on a function of the current state of thequeue when the classification is being calculated. The likelihood ofbeing in a particular state is a function of the offered load. Thisclassification function is configurable, and can be chosen withoutconstraint. For example, a given policer/shaper may be configured toassign one of two classifications to packets based on a probabilisticchoice. The classification probability used may be related to the lengthof the queue. That is, such a configuration may be designed to allocatehigher classifications to packets with a higher probability when theoffered load is low.

[0050] The classification of a packet determines, for instance, themaximum loss and delay it will most likely experience when it ismultiplexed with other streams at the output of the policer/shapers 104,as will be described further hereinbelow. This is a separate concept tothat of the loss and delay the packet may experience inside thepolicer/shapers 104. The loss experienced by a packet in thepolicer/shapers 104 depends on the recent arrival rate of the stream andthe length of the queue. The delay is determined by the configuredservice rates and the length of the queue.

[0051] Each of the policer/shapers 104 is preferably embodied using aqueue with a variable service rate. When a policer/shaper 104 receives apacket identifier, it determines whether the packet identifier should bestored in its internal queue or discarded. The control of the admissionof a packet identifier into a policer/shaper queue is discussed furtherhereinbelow.

[0052] In the following, the operation by which packets are admitted toor discarded from the policer/shapers 104 is first discussed. For thepurposes of this discussion, an example is taken of a policer/shaperutilizing a queuing system having a queue length of four as illustratedin FIG. 2. Each state in FIG. 2 is labelled with the length of the queuein that state. The service times in each state are obtained by samplingan exponential distribution with rate parameter “μ”. The service timesdetermine the conceptual rate of service for the packet. Two differentservice rates are used in the example shown in FIG. 2. If the queue isin state 1 (i.e., the queue has only one packet) then “μ₁” determinesthe rate of service for the packet at the head of the queue. If thequeue is longer, for example in state 3, service rate “μ₂” would beused.

[0053] This example embodiment of the policer/shapers 104 also hasarbitrary discard probabilities associated with each state. That is, onarrival, there is a probability that the packet identifier will bearbitrarily discarded. In this example, the probability of this eventdepends on the state of the queue when the packet identifier arrives.The operation of the policer/shapers 104 is now described in terms ofthe state of the queue on arrival of the packet identifier.

[0054] In a first case, it is assumed that the queue is full at the timethe packet identifier arrives. In such a case, the probability of thepacket identifier being entered into the queue is zero. This does notnecessarily mean that the packet is automatically discarded, as thepacket may have been sent to more than one of the availablepolicer/shapers 104. In such a case, the policer/shaper 104 forwards acommand on its respective line 138 a, 138 b, 138 c, 138 d, (which arecollectively referred to as line 138), to decrement the reference countfor the packet associated with a particular identifier. The queue memorymanager 114 then will discard the packet referenced by the packetidentifier if its reference count is 0 or less.

[0055] In a second case, at the time of arrival of a packet identifierat the policer/shapers 104 the queue is nearly full. For example,suppose the queue is in state 3. In state 3, there is a 30% chance thatthe packet identifier will not be entered into the queue. If the packetidentifier is admitted, then it is stored in the queue of thepolicer/shaper and the queue moves to state 4. The queue is then full,and any packet identifiers arriving before a packet identifier in thequeue departs will not be admitted.

[0056] If the packet identifier is entered into the queue of thepolicer/shapers 104, the policer/shapers 104 send an appropriate signalon its respective line 138 to the queue memory manager 114 indicatingthat the packet associated with the packet identifier is to be admittedinto the queue memory 112 and stored in a specific queue allocated forpackets belonging to this stream.

[0057] When a departure is scheduled, the packet identifier at the headof the queue is serviced. The sampled rate used to service this packetidentifier depends on the state of the queue. On the basis that thequeue is in state 4, the sample service rate used to service this packetidentifier is determined by an exponentially distributed random variablewith mean “μ₂”. The calculated service time of this packet is based onthe sample service rate and the length of the packet.

[0058] In a third case, a packet identifier arrives at a time when thequeue is nearly empty. The processing of the packet identifier in thiscase is very similar to the case when the queue is nearly full. Supposea packet identifier arrives when the queue is in state 1. There islittle chance of the packet being arbitrarily discarded, since theprobability of this event is configured to be zero. Therefore, thepacket identifier is stored in the queue, and the queue moves to state2. If a departure event is scheduled before another arrival occurs, thepacket identifier at the head of the queue is serviced based on theservice rate for state 2 which is “μ₁”. In a fourth case, the packetidentifier arrives at a time when the queue is empty. In this case, asin the third case, the packet identifier will be admitted to the queue.

[0059] The description contained hereinabove has assumed that thepolicer/shapers 104 are not configured to send any packets withoutdelay. In other words, the policer/shapers 104 preferably have no lowload transparency, all packets being delayed. In its simplest form, lowload transparency allows the first packet arriving at an empty queue tobe forwarded immediately. Notwithstanding this action, the queue movesinto state 1. Subsequent packets arriving are processed as though thispacket is present, except that when this packet would normally have beenforwarded on expiration of its timer, it is not sent. Whether or not apacket identifier has been forwarded immediately on arrival or not isrecorded in the queue. This concept can similarly be extended tomultiple packets.

[0060] Before a packet identifier leaves the policer/shapers 104, thepolicer/shapers 104 determines a quality classification for that packetidentifier. The policer/shapers 104 classify the packet identifiers, asdescribed hereinabove, with cherish and urgency classifications. Eachpacket identifier should be classified with a cherish and urgencyclassification before it is forwarded to the cherish/urgency multiplexor106. The classification assigned to a packet identifier is a function ofthe current state of the queue.

[0061] Again, assuming the policer/shapers 104 have a queue of length 4.As shown in FIG. 2, it is assumed that the queue of the policer/shapers104 is configured such that the service rates for states 1 and 2 aredefined as μ₁, and the arbitrary loss possibility for states 1 and 2 iszero. States 3 and 4 have a defined service rate of μ₂, and have anarbitrary loss probability of 0.3 (i.e., 30%).

[0062] In a preferred embodiment, the policer/shapers 104 are configuredwith a primary and a secondary cherish/urgency classification and apacket identifier is assigned one of the classifications upon arrival.Each state has an associated probability of a packet being classifiedwith the primary classification. The probability of classifying a packetwith the primary classification in each state is, in this example,configured to be: for state 1, 100%; for state 2, 80%; for state 3, 60%;and for state 4, 40%. These probabilities are illustrated in FIG. 3.

[0063] For example, the primary classification may be a more desirableclassification than the secondary classification. If the offered load onthe stream is low, the packets have a higher probability of beingassigned the more desirable primary classification, as the queue willmost often be in state 1 or 2. As the offered load increases, there is ahigher probability that a packet identifier will be assigned thesecondary classification, which is less desirable, as this could meanthe packet identifier will experience more delay and/or loss in thecherish/urgency multiplexor 106.

[0064] Referring to the discussion hereinabove of the criteria foradmitting a packet identifier to the queue, when a packet identifier isadmitted to the queue in case 2 (i.e., in the example when the queue isin state 3 and the packet identifier is admitted, moving the queue tostate 4) the probability of classifying the packet identifier with theprimary classification is 40% as the queue is then in state 4.Therefore, there is a 40% chance that the packet identifier will beassigned the primary classification, and a 60% chance that it will beassigned the secondary classification. This classification of the packetidentifier is based on a simple probabilistic choice.

[0065] A packet identifier is then emitted from the policer/shapers 104queue at the end of the calculated service time, and the queue moves tostate 0.3. A new calculated service time based on the defined parametersof state 3 then determines when the next departure is performed. If anarrival occurs before this period of time expires, the queue may thenmove to state 4 again, based on whether the arrival is arbitrarilydiscarded or not.

[0066] Referring to case 3 above, in the case where a packet identifierarrives when the queue is nearly empty, the packet identifier isclassified using the probability associated with state 2. In this case,there is an 80% chance of the packet identifier being sent to thecherish/urgency multiplexor 106 with the primary classification.

[0067] Arbitrarily discarding packets on their arrival at thepolicer/shapers 104 not only reduces the arrival rate of the stream, butalso helps to avoid burst loss. For example, if a burst of six packetsarrived in the queuing system of FIG. 2, the last two packets would belost if packets were not arbitrarily discarded. On the other hand, ifthe probability of arbitrary discard increases with the queue length, itmay be the case that, for example, the fourth or third packet isdiscarded on arrival, thus distributing the loss in the burst morefairly.

[0068] Another function of the policer/shapers 104 as describedhereinabove, is to keep the packet associated with the packet identifierbeing processed in the queue memory 112 by interacting with the queuememory manager 114. Storing packets in queues is preferable to ensurethat packets in a stream are not re-ordered by the service process ofthe cherish/urgency multiplexor 106. Depending on the function chosen toassign cherish/urgency levels to packets, there is the possibility thepackets could be re-ordered during the multiplexing onto line 124, asdiscussed further hereinafter.

[0069] As an example, consider the simple probabilistic classificationfunction described above. Assume a burst of four packet identifiersarrive in one of the policer/shapers 104 and all packets are stored inthe queue. Furthermore, assume no more arrivals occur during the firstperiod in question. Furthermore again, assume that the cherish/urgencymultiplexor 106 is not empty, with packets originating from other of thepolicer/shapers 104. Finally, assume that the primary cherish/urgencyclassification has a desirable high urgency level, while the secondaryclassification has a low urgency level. Given these conditions, thepacket identifier at the start of the burst, referred to as “packetidentifier 1” has a higher probability of being assigned the secondarycherish/urgency classification then a packet near to the end of theburst, called “packet identifier 4.” If packet identifier 1 is assignedthe secondary classification, while packet identifier 4 is assigned theprimary classification, the difference in urgency levels may causepacket identifier 4 to be serviced before packet identifier 1 in thecherish/urgency multiplexor 106.

[0070] In order to avoid this, the policer/shapers 104 instruct thequeue memory manager 114 to queue packets in the queue memory 112according to the order in which the corresponding packet identifiers arereceived in the policer/shapers 104. That is, on arrival in thepolicer/shapers 104, if a packet identifier is not discarded, the queuememory 112 is instructed to queue the packet in the queue of therelevant stream in the order in which it arrived.

[0071]FIG. 8 illustrates a block diagram of an embodiment of apolicer/shaper, generally designated 700, constructed in accordance withthe principles of the present invention. The policer/shaper 700 includesa policer/shaper arrival process block 701, a packet identifier queueblock 702, a timer 703, a policer/shaper departure process block 704, apolicer/shaper configuration management block 705, a discard probabilitygenerator 706, and a service time generator 707.

[0072] Packet identifiers arrive via line 708 to the policer/shaperarrival process block 701. The policer/shaper arrival process block 701notifies the policer/shaper configuration management block 705 via line720 that a packet identifier has arrived. The policer/shaperconfiguration management block 705 polls the packet identifier queueblock 702 via line 724 to obtain the current length of the queue withinthe policer/shaper 700. Based on the response from the packet identifierqueue block 702 via line 726, the policer/shaper configurationmanagement block 705 determines if the queue is full or not. If there isavailable capacity in the queue, the policer/shaper configurationmanagement block 705 then determines whether or not the packetidentifier should be arbitrarily discarded, using input from the discardprobability generator 706 on line 732.

[0073] If the packet identifier is to be admitted to the queue, thepolicer/shaper configuration management block 705 alerts thepolicer/shaper arrival process block 701 via line 722 to admit thepacket identifier. On receipt of this response, the policer/shaperarrival process block 701 sends a request via line 736 (equivalent toone of lines 138) to a queue memory manager 114 to enqueue the packet inthe one queue in the queue memory 112 which is allocated for thisstream. The policer/shaper arrival process block 701 then forwards thepacket identifier to the packet identifier queue block 702 via line 710.The policer/shaper configuration management block 705 calculates a newservice time, based on input from the service time generator 707 on line734 and the length of the packet, and sends this service time to thepolicer/shaper departure process block 704 via line 728. Thepolicer/shaper departure process forwards the new service time to thetimer 703 via line 716. The timer 703 resets itself to wake up at theend of the new service time.

[0074] If the policer/shaper configuration management block 705determines that the queue is full, it instructs the policer/shaperarrival process block 701 via line 722 to discard the packet identifier.In this case, the policer/shaper arrival process block 701 sends adiscard instruction to the queue memory manager 114 via line 736. Thepolicer/shaper arrival process block 701 then discards the packetidentifier.

[0075] When the timer 703 wakes up, it sends a request via line 718 tothe policer/shaper departure process block 704 to emit a packetidentifier. The policer/shaper departure process block 704 sends arequest to the policer/shaper configuration management block 705 vialine 730 for a classification and a new service time. The policer/shaperconfiguration management block 705 polls the policer/shaper packetidentifier queue block 702 via lines 724 and 726 to obtain the queue'scurrent length. The policer/shaper configuration management block 705uses the current length of the queue to determine the classification forthe packet identifier which is about to be emitted. The classificationis sent to the policer/shaper departure process block 704 via line 728.The policer/shaper departure process block 704 concatenates a queueidentifier, specifying which queue in the queue memory 112 is used forstoring packets of this data stream, and the classification to thepacket identifier and forwards this tuple of data on line 738.

[0076] It should be noted that in this example implementation theclassification of the packet identifiers is carried out as the packetsleave the queue. The point at which the packets are classified withinthe policer/shapers 104 is, however, implementation dependent, and isnot limited to the example given herein. As described hereinabove, thepacket identifiers may be classified on arrival rather than ondeparture.

[0077] If the queue identified in the packet identifier queue block 702is non-empty, the policer/shaper configuration management block 705 alsosends a new service time to the policer/shaper departure process block704 via line 728. This service time is forwarded to the timer 703 vialine 716, and the timer 703 sets itself to wake up after this time. Ifthe queue is empty, no action is taken.

[0078] Further possible modifications to the policer/shapers 104 shownin FIG. 1 are illustrated with reference to FIG. 6. As shown in FIG. 6,the policer/shapers 104 may be cascaded in various ways.

[0079] Referring to FIG. 6(a), there is illustrated an advantageousarrangement in which the output of a policer/shaper 210 provides aninput to two parallel policer/shapers 212, 214. The policer/shaper 210receives packet identifiers on line 222 from the stream identifier 102.In accordance with the standard operation of the policer/shapers 104,the policer/shaper 210 selectively discards packet identifiers andtime-shifts packet identifiers to provide a modified flow of packetidentifiers on its output. In a first embodiment, the modified flow ofpacket identifiers is broadcast on lines 234 and 236 to each of theparallel policer/shapers 212, 214. The respective policer/shapers 212,214 then selectively discard packet identifiers and time-shift packetidentifiers to generate two further modified packet identifier flows ontheir outputs 224 and 226. In a second embodiment, the output of thepolicer/shaper 210 is selectively provided to either one or the other ofthe policer/shapers 212 and 214. The appropriate one of thepolicer/shapers 212, 214 then selectively discards packet identifiersand time-shifts packet identifiers onto its respective output.

[0080] Referring to FIG. 6(b), there is illustrated a furtherarrangement in which two parallel policer/shapers 216, 218 receiveinputs including packet identifiers on flows on lines 228 and 230. Theoutput of both policer/shapers 216, 218 on their respective outputs 238and 240 form an input to a further policer/shaper 220. Thepolicer/shaper 220 outputs packet identifiers on line 232.

[0081] It will be apparent to the person skilled in the art howpolicer/shapers can be cascaded in various combinations of thearrangements shown in FIG. 6. For example, the arrangements of FIGS.6(a) and 6(b) may be cascaded. In addition, any one of thepolicer/shapers may also receive inputs from other sources. For example,the policer/shaper 212 of FIG. 6(a) may receive an additional inputwhich is not derived from another policer/shaper. One skilled in the artwill appreciate how various cascading arrangements may be implemented.The only constraint is that the cascaded policer/shapers should beconnected in an acyclic graph.

[0082] On departure, the policer/shaper sends both the packetidentifier, and its associated queue identifier and classification tothe cherish/urgency multiplexor 106 via a respective output line 122.The cherish/urgency multiplexor 106 employs the packet identifier inaddition to the queue identifier in case it has to issue a discardinstruction to the queue memory manager 114, as will be discussed infurther detail hereinbelow.

[0083] The cherish/urgency multiplexor 106 manages the contention forthe network resources between two or more streams. The cherish/urgencymultiplexor 106 receives packet identifiers from the variouspolicer/shapers 104, each packet identifier being tagged with acherish/urgency classification.

[0084] The classification of a packet identifier defines a cherish leveland an urgency level for the packet with which it is associated. Thecherish/urgency multiplexor 106 manages the contention between two ormore streams by servicing packets (via their packet identifiers)depending on their urgency level and, when necessary, discarding packetsdepending on their cherish level. The cherish level determines whichpackets are entered into the cherish/urgency multiplexor 106. Theurgency level determines the order in which the packets are taken fromthe cherish/urgency multiplexor 106. Classifying packets using cherishand urgency levels is described in International Patent Application No.PCT/GB00/01569. Thus the cherish/urgency mulitplexor 106 manages thecontention between the three streams on the output lines 122 a, 122 band 122 c at the outputs of the policer/shapers 104 a, 104 b and 104 c.

[0085] When a packet identifier with its associated cherish/urgencyclassification arrives, the cherish/urgency multiplexor 106 determineswhether the packet identifier should be stored or discarded. This isdetermined by the available storage capacity in the cherish/urgencymultiplexor 106 and the cherish level associated with the identifier.

[0086] In one known method of implementing cherishing, access to abuffer in the cherish/urgency multiplexor 106 is determined byassociating a cherish level with a particular area of the buffer. Thatis, suppose a packet can be classified with any one of “N” cherishlevels. Then, assume there are “K_(N)” buffers in total. Traffic ofcherish level 1 can enter any of the “K_(N)” buffers, while traffic ofcherish level “i” can only enter the first K_(N−i+1) buffers, whereK₀=0<K₁< . . . <K_(N). Therefore, packets of cherish level 1 will havegreater probability of finding available buffer resources than packetsof cherish level “i,” where i>1.

[0087] A scheme is also necessary to determine which packet identifierto forward, that is, how the urgency levels should be used to determinethe order in which packets are serviced. One approach is to forward theidentifier at the head of the queue with the highest urgency level,although other approaches are possible. The loss rate for packetidentifiers in a given stream is determined both by the cherish levelassigned to the packets and by the maximum arrival rate for all otherstreams which have a cherish level of equal or greater value. The orderin which packets are serviced in the cherish/urgency multiplexor 106depends on their urgency levels and the arrival rates and patterns ofmore urgent traffic. This determines the delay experienced by the packetidentifier in the cherish/urgency multiplexor 106.

[0088] If the cherish/urgency multiplexor 106 discards a packetidentifier it instructs the queue memory manager 114 via line 136 todecrement the reference count for the appropriate packet from the queuememory 112. The cherish/urgency multiplexor 106 records statistics aboutthe traffic flows, including but not limited to, the number of the bytesand/or the number of packets both accepted and rejected for each urgencylevel and each cherish level.

[0089] The cherish/urgency multiplexor 106 forwards one of the storedpacket identifiers along with its associated queue identifier from itsinternal buffers to the rate limiter 108 on line 124 responsive to arequest from the rate limiter 108 on line 126. The choice of whichidentifier pair is forwarded to the rate limiter 108 responsive to arequest therefrom is determined by the packet identifier's urgency leveland the selection mechanism of the cherish/urgency multiplexor 106.

[0090] Turning now to FIG. 9, illustrated is a block diagram of anembodiment of a cherish/urgency multiplexor, generally designated 800,constructed in accordance with the principles of the present invention.The cherish/urgency multiplexor 800 includes a cherish/urgency arrivalprocess block 801, a set of urgency queues 814 comprising queues802-805, a cherish/urgency departure process block 806, and acherish/urgency configuration management block 807.

[0091] Packet identifiers arrive via line 810 to the cherish/urgencyarrival process block 801. The cherish/urgency arrival process block 801notifies the cherish/urgency configuration management block 807 that apacket identifier has arrived and forwards its cherish level via line822. The cherish/urgency configuration management block 807 requests thelength of each one of the urgency queues 802-805 via line 826. Based onthe current total number of packet identifiers queueing in thecherish/urgency multiplexor 800 and the cherish level of the packetidentifier, the cherish/urgency configuration management block 807determines whether or not to discard the packet.

[0092] If the packet identifier is to be discarded, the cherish/urgencyconfiguration management block 807 notifies the cherish/urgency arrivalprocess block 801 accordingly via line 824. In this case, thecherish/urgency arrival process block 801 sends an instruction todiscard the packet to the queue memory 112 via line 812, identifying thepacket to discard by its packet identifier and its queue identifier.These identifiers are then discarded by the cherish/urgency arrivalprocess block 801.

[0093] Otherwise, the cherish/urgency configuration management block 807notifies the cherish/urgency arrival process block 801 via line 722 toforward the packet identifier to one of the urgency queues 802-805.There is one of the urgency queues 802-805 for each urgency level. Thisexample implementation illustrates a cherish/urgency multiplexor 800configured for four possible urgency levels. The cherish/urgency arrivalprocess block 801 obtains the urgency level for the packet identifierand forwards the packet identifier and its associated queue identifiervia one of the lines 830 to the appropriate one of the urgency queues802-805.

[0094] Requests for packets from the rate limiter 108 are received bythe cherish/urgency departure process block 806 via line 818. When apacket request arrives, the cherish/urgency departure process block 806requests the head element of one of the urgency queues 802-805 via oneof the lines 834. In the preferred embodiment, the identifiers at thehead of the most urgent of the urgency queues 802-805 will be requested.The packet identifier and queue identifier pair at the head of the queuewhich has received the request are forwarded to the cherish/urgencydeparture process block 806 via one of the lines 832. Thecherish/urgency departure process block 806 forwards the pair ofidentifiers immediately to the rate limiter 108 via line 124.

[0095] The rate limiter 108 moves contention from a point downstream inthe network to within the multiplexor 100 by restricting the servicerate to one for which the network has sufficient resources. The ratelimiter 108 ensures that the maximum service rate is not exceeded in thelong term by assuring that appropriate gaps are maintained betweentransmitted packets.

[0096] The rate limiter 108 requests packet identifiers from thecherish/urgency multiplexor 106 on a request line 126, and receivespacket identifiers along with their associated queue identifiers on line124. On receiving the pair of identifiers on line 124, the rate limiter108 provides the queue identifier on line 134 to the queue memorymanager 114. Responsive thereto, the queue memory manager 114 providesthe packet at the head of the identified queue from the queue memory 112to the rate limiter 108 on line 132. The rate limiter 108 then forwardspackets for transmission to the output interface 110 on line 128. Onforwarding a packet to the output interface 110, the rate limiter 108sets a timer to represent the service time of the particular packet atthe configured rate. At the end of the timing period assigned to theservicing of the particular packet, the rate limiter 108 requests afurther packet on line 126 from the cherish/urgency multiplexor 106.

[0097] If no re-ordering has occurred in the cherish/urgency multiplexor106, the packet sent from the queue memory manager 114 will be the samepacket identified by the packet identifier. Otherwise, the packetidentifier received by the rate limiter 108 will refer to a packet whichis still waiting in the queue in the queue memory 112.

[0098] The rate limiter 108 can service packets stochastically ordeterministically. The rate limiter 108 may thus service packets suchthat they have variable spacing, as discussed hereinabove with referenceto the policer/shapers 104.

[0099] A unit can be made from combining a cherish/urgency multiplexor106 and rate limiter 108, preferably one which services packetsstochastically. Such units can be cascaded with a plurality ofpolicer/shapers and other such cherish/urgency multiplexor and ratelimiter units and may receive additional inputs from other sources. Oneskilled in the art will appreciate how various cascading arrangementsmay be implemented. The only constraint is that the cascaded combinationof these units should be connected in an acyclic graph.

[0100] If stochastic service rates are used, there is the potential thatthe sampled service rate may be much faster than the rate at which thepacket can be physically transmitted by the output interface. In thiscase, the calculated resource size of the packet will be smaller thanthe size of the packet as illustrated in FIG. 5. Referring to FIG. 5a,there is illustrated the size of the packet, according to the physicaltransmission rate of the transmission medium to which it will beforwarded. FIG. 5b illustrates the size of the packet where the servicerate chosen for the packet is greater than the rate at which the packetwill actually be transmitted on the physical medium. FIG. 5c illustratesthe size of the packet in the case where the service rate chosen for thepacket is smaller than the rate at which the packet will actually betransmitted by the output interface 110.

[0101] As can be seen from FIG. 5, if the calculated resource size isactually smaller than the actual resource size of the packet, twopackets may overlap. In a practical system two packets should notoverlap during transmission. Instead, a burst of two or more packetswill be observed. Therefore, when the packet identifiers are servicedstochastically, there is a finite probability that two or more packetswill be sent back-to-back.

[0102] The output interface 110 provides flow control feedback on line130 to the rate limiter 108. If the output interface 110 indicates tothe rate limiter 108 that transmission has been suspended, the internaltiming mechanism of the rate limiter 108 is affected. In this case, therate limiter 108 can perform one of several actions. For example, therate limiter 108 may be configured to discard any packet which isscheduled for servicing during the suspension period. Such discarding ofpackets by the rate limiter 108 causes the rate limiter 108 to generatean alarm signal.

[0103] The rate limiter 108 can also be improved in an attempt tomaximize the resource of the external system at a low load. That is,under low loads, the service rate can be chosen which is faster then theconfigured rate. As the load on the system increases, the service ratewill be decreased until this is near to or at the configured rate.

[0104] The output interface 110 provides the basic functionalitynecessary for the onward transmission of data packets. The outputinterface 110 provides flow control feedback on lines 130 to the ratelimiter 108 when some external back pressure (for example, from thetransmission medium) occurs. Thus, the output interface 110 transmitspackets on line 142, and receives flow control signals on line 144 fromthe operating system or device driver.

[0105] The output interface 110 is responsible for little directprocessing relating to the quality management of the data stream. If apacket is received by the output interface 110 and the external systemor device indicates that it has available resources for transmission,then the packet is transmitted by the output interface 110 withoutdelay. If the rate limiter 108 is operating in deterministic mode andflow control has not been exerted, there is never more than one packetin the buffer of the output interface 110 at any one time.

[0106] Thus, there has been described an invention which may beadvantageously utilized in a multiplexor for providing a predeterminedquality of service. Although the present invention has been described indetail, those skilled in the art should understand that they can makevarious changes, substitutions and alterations herein without departingfrom the spirit and scope of the invention in its broadest form.

What is claimed is:
 1. A method of allocating a priority level in aninformation flow in a data transmission system, comprising: receiving aplurality of data packets; and allocating a priority level to each datapacket by determining a class of loss for the data packet, a class ofurgency of service for the data packet, and further in accordance with acurrent load of the information flow.
 2. The method as recited in claim1 further comprising generating a modified information flow by providinga variable spacing between the data packets.
 3. The method as recited inclaim 2 wherein the variable spacing between data packets is random. 4.The method as recited in claim 2 wherein the variable spacing betweendata packets is exponentially distributed.
 5. The method as recited inclaim 1 wherein a spacing between at least some of the data packetscorresponds to a spacing in the information flow.
 6. The method asrecited in claim 1 wherein the act of allocating the priority level isfurther in accordance with a state of a queue that the data packets areselectively admitted.
 7. The method as recited in claim 1 furthercomprising selectively admitted each data packet to at least one of aplurality of queues and multiplexing an output of the queues.
 8. Themethod as recited in claim 1 further comprising selectively discardingdata packets.
 9. The method as recited in claim 1 further comprisingallocating a data packet identifier to each data packet and storing eachdata packet in a sequence.
 10. The method as recited in claim 1 furthercomprising servicing each data packet based on the priority level andoutputting each data packet in a sequence.
 11. A controller forallocating a priority level in an information flow, comprising: meansfor allocating a priority level to each data packet includingdetermining a class of loss for the data packet and determining a classof urgency for the data packet, the priority level further being inaccordance with a current load of the information flow.
 12. Thecontroller as recited in claim 11 further comprising means forgenerating a modified information flow by providing a variable spacingbetween the data packets.
 13. The controller as recited in claim 12wherein the means for generating a modified information flow is embodiedat least in part in a policer/shaper.
 14. The controller as recited inclaim 12 wherein the means for generating a modified information flowemploys one of a random pattern and a pseudo-random pattern.
 15. Thecontroller as recited in claim 11 further comprising means forselectively admitting a data packet to a queue.
 16. A controller forallocating a priority level in an information flow, comprising: apolicer/shaper configured to allocate a priority level to each datapacket including determining a class of loss for the data packet anddetermining a class of urgency for the data packet, the priority levelfurther being in accordance with a current load of the information flow.17. The controller as recited in claim 16 wherein the policer/shaper isconfigured to generate a modified information flow by employing avariable spacing between the data packets.
 18. The controller as recitedin claim 17 wherein the policer/shaper employs one of a random patternand a pseudo-random pattern.
 19. The controller as recited in claim 16further comprising queue memory manager configured to selectively admita data packet to a queue.
 20. The controller as recited in claim 16wherein the controller is configured to generate a plurality ofinformation flows, the controller including a cherish/urgencymultiplexer for multiplexing the information flows into a cascadedarrangement.